One of the most promising ways of improving the wavelength-discriminating ability of optical filters, i.e. increasing the finesse of the filter, has been to pass the light through a cascade of several etalons having incommensurate spacings of their transmission peaks and each having only modest finesse. To make such structures electrically tunable, it is the usual practice to provide some sort of piezoelectric structure that changes the mirror spacings by the application of one or more electrical voltages.
There are two problems with such approaches. First, the tuning speed is limited by the inertia of the physically moving parts. Second, the various parts of the assembly are subject to dimensional variations due to temperature changes and mechanical vibration, so that elaborate means must be taken to stabilize them. Although it is quite practical to make single or double-cavity etalons with a finesse of several thousand, as would be required for a wavelength-division communication network of, say, one thousand stations, the limitations on tuning speed and stability remain a serious problem with this class of solutions.
It is well known in the current art of tunable multicavity etalons that if a solid wedge of glass with reflective surfaces has a gradual enough taper, one can form a tunable etalon by controlling the position along the taper at which the light passes through the wedge.
Single- and multi-cavity etalons are described in the book The Fabry-Perot Interferometer, by J. M. Vaughan, published by the American Institute of Physics and herein incorporated by reference for its teachings in the field of optics. The use of separately tuned two-cavity etalons in optical communication applications is described in the paper by I. Kaminow, P. P. Iannone, J. Stone and L. W. Stulz, A Tunable Vernier Fiber Fabry-Perot Filter for FDM Demultiplexing and Detection, IEEE Photonics Tech. Ltrs., Vol 1, No. 1, January 1989, pp. 24-25, herein incorporated by reference for its teachings in the field of optics.
The use of wedges goes all the way back to Fizeau (see Born and Wolf, Principles of Optics, Pergamon Press, 6th Edition, p. 289-290). However, if the two surfaces of the wedge have only low reflectivity, interference takes place between only two weakly reflected beams of light, one from the near surface and one from the far surface. In a true wedge etalon, both facing surfaces of the wedge are reflective-coated so that the light makes many bounces, as in any Fabry-Perot interferometer. The use of wedge etalons as optically tunable devices has been described in the book by S. Tolansky, An Introduction to Interferometry, Longmans, Green and Co., 1955, and by N. Barakat, M. Medhat, H. A. El-Hennawi and S. Y. El-Zaiat in Methods of Controlling the Spatial Frequency of the Fringes Formed by a Wedge Interferometer, Optik, Vol. 81, No. 1, 1988, pp. 1-5, herein incorporated by reference for its teachings in the field of optics.
Acoustooptic deflectors, in which a high-frequency acoustic wave effectively sets up a grating in a crystal, have been used for steering laser beams since the mid-1960s. It is clear that such a deflector followed by a pinhole passing only one diffracted angle of light could be used for a crude tunable optical filter, but the number of resolvable wavelengths for such a scheme is quite small. Comprehensive discussions of acoustooptic deflectors and filters have been given by A. Korpel in "Acoustooptics--A Review of Fundamentals", Proc. IEEE, Vol. 69, No.1, January 1981, pp. 48-53, and by I. C. Chang in "Acoustooptic Devices and Applications", IEEE Transactions on Sonics and Ultrasonics, Vol. 23, No. 1, January, 1976, pp. 1-22, herein incorporated by reference for its teachings in the field of optics.
Thus, numerous references disclose the use of single-cavity etalons or acoustooptic deflectors alone to build tunable optical filters. In addition, several references have combined these two technologies (K. Liu and F. Tong, Acousto-optically Tuned Angle Etalon, IBM Tech. Disclosure Bull., Vol. 32, No. 10B, March, 1990, p. 441, herein incorporated by reference for its teachings in the field of optics; U.S. Pat. No. 4,204,771 to Shull et al.). In these references, a deflector, which precedes a single plane-parallel etalon, is used simply to change the angle at which the light passes through the etalon, thus changing slightly the resonance wavelength. The tuning range is very small as a result of the etalon being plane-parallel rather than wedge shaped.
U.S. Pat. No. 3,914,055 to Wolga et al. teaches a combination of an acoustooptic filter (as opposed to a deflector) and an etalon. The filter does rough filtering and the etalon does fine filtering.
Deflection, followed by a single wedge, is described in the paper by W. Baer and A. Gardner, "A Scanning Fabry-Perot Interferometer", Journal of the Optical Society of America, Vol. 51, pp. 1400-1401. However, this device lacks speed and selectivity because the deflection is mechanical and only a single wedge etalon is used.